An organic electroluminescent device (below, an organic electroluminescent device is referred to as “an organic EL device”) is attracting attention, and study and development are being made of the organic EL device as a light emitting source of the next generation, because the organic EL device can be made small easily, has low power consumption, can be fabricated to be a surface light source, requires a much lower power voltage than that of a liquid crystal device, and hence can be used in a flat display and other various display devices.
FIG. 1 is a cross-sectional view of an organic EL device of the related art. As illustrated in FIG. 1, an organic EL device 10 of the related art includes an anode 12 formed from transparent ITO (Indium Tin Oxide), a hole transportation layer 13, a light emission layer 14, an electron transportation layer 15, and a cathode 16; these layers are stacked sequentially on a transparent glass substrate 11. The hole transportation layer 13 transports holes to the light emission layer 14 efficiently, increases spatial electron density, and increases light emission efficiency. In addition, it has been proposed to provide a layer between light emission layers to block electrons moving between the light emission layers, so as to improve the light emission efficiency of the light emission layer.
FIG. 2 is an energy diagram of the organic EL device 10 in FIG. 1. When a voltage is applied to the organic EL device 10, holes 22 move from the anode 12 to the light emission layer 14. Due to movement of the holes 22, an electric current of the holes 22 is generated, and due to movement of electrons 21, an electric current of the electrons 21 is generated. When the electrons 21 and the holes 22 arrive at the light emission layer 14, the electrons 21 and the holes 22 re-combine, and release energy. Due to the released energy, organic fluorescent materials in the light emission layer 14 are excited and emit light.
Efficiency of light emission is expressed by light emission luminance. Hence, the efficiency of the light emission of the light emission layer 14 is determined by the magnitudes of the hole current and the electron current, which flow through the organic EL device 10, and when either of the currents is large, the light emission efficiency of the light emission layer 14 declines. The magnitudes of the hole current and the electron current depend on types of the films constituting the organic EL device and film thicknesses of the stacked films. For example, if the magnitude of the hole current is unduly larger than the electron current, there arises a problem in that an increasing portion of the hole current is consumed but does not contribute to light emission, and this results in reduction of the light emission efficiency of the light emission layer 14, and an increase of power consumption.